A bobbin is a component of a sewing machine that holds a spool of thread on which a thread is wound thereinto, and in spinning fields, the bobbin is a spindle or cylinder on which a rough thread or twisted thread is wound, which may be made of wood or fiber.
Generally, a sewing machine is provided with an upper thread that is inserted into the eye of a needle and supplied along the needle on the top of fabric and a lower thread that is supplied from the underneath of the fabric for conducting over locking. The lower thread is wound on a separate bobbin, and the bobbin is used in the state of being embedded into a rotating bobbin case disposed on the underside of a table of the sewing machine, thus making it impossible to supply the lower thread continuously. If the lower thread is consumed up, accordingly, the bobbin should be exchanged with new one to supply the lower thread continuously, thus restarting the sewing work. On the other hand, a device has been developed to recognize the remaining amount of the lower thread and thus to inform a user of the exchange time, which undesirably increases the manufacturing cost thereof.
Further, a maximum amount or thickness of the lower thread really wound on the bobbin is set to a maximum amount of the lower thread that is wound at the close position to a position where a sewing needle is descended and does come into contact with the sewing needle during the sewing work. Accordingly, the amount of the lower thread wound on the conventional bobbin is very limited. As a result, the bobbin should be exchanged frequently with new one by a user or sewing machinist, which undesirably decreases the efficiency of the sewing work.